Workflow is trigger even if startup condition is not met

Hi,

I'm using a sharepoint 2013 nintext workflow that is trigger on the modification of items in a list. I have setup this workflow option as 'Start when items are modified' to Conditional. In the condition I have written an impossible condition to be met ( StatusField = Completed AND StatusField = Failed ) - StatusField is a lookup column in my list. The Workflow is run each time a item is modified, regardless of the value of the StatusField. 

How can this be possible ?

I don't want the workflow to be trigger on each modification of the item since I have an external system that modify this item each few seconds to set a progression stage on it. So I don't want the workflow to be run each n seconds, just want to run it at the end.